1. Field of Invention
This invention relates to an electromagnetic flow meter for metering the flow rate of a fluid by applying a magnetic field thereto and, more particularly, to such a flow meter having improved excitation and signal processing.
2. Description of the Prior Art
Heretofore, regular commercial power with commercial frequency has been used for excitation of an electromagnetic flow meter. This commercial frequency type exciting method has certain advantages. For example, (a) it has a quick response at reasonable production cost, and (b) it is not substantially susceptible to influences due to random noises (which will hereinafter be referred to as "flow noises") generated in a fluid of slurry or low conductivity and having a frequency increasing with the flow velocity. However, disadvantageously, this method produces fluctuations of the zero point when the flow meter remains in a running state for any relatively long periods of time, e.g. one day.
There also exists a low frequency exciting method wherein the frequency is one-half or lower than the commercial frequency. This low frequency exciting method, advantageously, has a relatively stable zero point. But, disadvantageously, the low frequency is close to the frequency of the flow noises so that susceptibility to influences due to the flow noise is increased, and becomes more pronounced as the flow velocity gets higher. Moreover, damping is needed to reduce flow noise influence and such damping causes delays in response.
Furthermore, recently developed flow meters tend to reduce electric power. This power reduction is now required of two wire type electromagnetic flow meters which conduct power from the source and transmit signals through the two lines. In this case, the electromotive force per unit flow velocity has to be reduced. In the low frequency exciting method, for example, the electromotive force of about 0.5 mV/m/s is reduced to 10 mV/m/s for the two line type. If the electromotive force generated is made lower by one order or more than that of the prior art, the influences due to the flow noises will be relatively augmented and hence require raising of the limit to the power economy of the low frequency excitation method.
Thus, to summarize, excitation using commercial frequency is advantageous in that quick response is obtained and there is low susceptibility to flow noises. On the other hand, use of commercial frequency excitation produces an unstable zero point. Using low frequency excitation gives a stable zero point, but is more susceptible to influences due to flow noises. Thus, as can be appreciated, adoption of either prior methods is accompanied by problems.